The tendency for a soil to lose its nutritive properties and therefore to go through a certain form of degradation is related mainly to its low water retention efficiency and to its slow but noticeable chemical changes which are caused by the action of various external factors such as acid rain.
The use of synthetic and natural zeolites as soil conditioners has been studied extensively over the last few years. In fact, apart from industrial applications as selective adsorbents and catalysts, their use as soil conditioners is one of the most important applications recently developed.
The zeolites not only improve the physical properties of the soil, but their postulated mode of action in crop growth exploits the ion-exchange capabilities for the nutritive ions such as NH.sub.4.sup.+ and K.sup.+. Thus, the high exchange and absorption capacity and the retention of the absorbed ions create nutritive and humidity conditions around the roots which are beneficial to the crop.
The zeolite action is therefore that of a solid material which by virtue of its physico-chemical properties enhances the efficiency of classical fertilizers. It is likely that the primary nutrient ions in fertilizers are initially taken up by the zeolite framework, which then gradually releases these ions to the soil and the roots of the plants. More particularly, the reversible cation-exchange capability of the zeolites permits overcoming of phenomena such as the washing away of fertilizers during irrigation, when the water-soluble primary nutritive components are rapidly lost, leading to a decrease in efficiency. Furthermore, because of the good water adsorption properties of the zeolites, the roots of the crops are provided with appropriate humidity conditions for an extended period of time.
To summarize, zeolitic materials show a great retention capacity for water and other soluble organic and inorganic compounds, thus favoring better growth of a large number of vegetal species. Therefore, because of their chemical properties and their anti-abrasive resistance, these zeolitic materials, once added to the soil, constitute long lasting soil conditionters. Because of their good ion-exchange properties, various chamical compounds such as water, compounds containing water-soluble elements such as nitrogen, potassium, or other water-soluble inorganic cations such as potassium, calcium, manganese, copper, iron and magnesium among others can be pre-incorporated into the zeolitic material and/or can be ion-exchanged into the zeolitic lattice from the soil solution.
Another interesting property of the zeolites is the fact that they possess a rigid framework which prevents them from undergoing structural condensation upon severe drying conditions as is frequently the case for commonly used clays. The latter, when collapsed, trap the ions present in their structures, thus preventing any further supply to the plant roots.
Promising results were obtained for most of the zeolites which were the object of investigations. In fact, it was found that an average increase of approximately 50% in the degree of moisture of normal soil could be sustained for 8 to 12 days by incorporating a zeolite material. However, all the commercial zeolites studied to date were found to possess an interesting but limited water-retention capacity.